The present invention relates generally to error detector circuits and, more particularly, to an error detector circuit, and associated method, for a receiver operative to receive a discretely-encoded signal.
A communication system is comprised, at a minimum, of a transmitter and a receiver interconnected by a transmission channel. A communication signal generated by the transmitter is transmitted upon the transmission channel, thereafter to be received by the receiver.
A two-way communication system is a communication system which permits both transmission and reception of communication signals between at least two locations. Two-way communication between the at least two locations is thereby permitted.
A radio communication system is a communication system wherein the transmission channel comprises a radio frequency channel. The radio frequency channel is defined by a range of frequencies of the electromagnetic frequency spectrum. A transmitter operative in a radio communication system converts a communication signal which is to be transmitted into a form suitable for transmission thereof upon the radio frequency channel.
Conversion of the communication signal into the form suitable for the transmission thereof upon the radio frequency channel is effectuated by a process referred to as modulation. In such a process, the communication signal is impressed upon an electromagnetic wave. The electromagnetic wave is commonly referred to as a "carrier signal." The resultant signal, once modulated by the communication signal, is referred to as a modulated carrier signal or, more simply, a modulated signal. The transmitter includes circuitry to perform such a modulation process.
Because the modulated signal may be transmitted through free space over large distances without the requirement of a fixed connection between the transmitter and the receiver operative in such a radio communication system, radio communication systems are oftentimes utilized to effectuate communication between a transmitter and a remotely-positioned receiver.
The receiver of a radio communication system which receives the modulated signal contains circuitry analogous to, but operative in a manner reverse with that of, the circuitry of the transmitter and is operative to perform a process referred to as demodulation.
A radio transceiver is a device which includes both a radio transmitter and a radio receiver to permit thereby two-way communication therethrough.
Improvements in radio telephony techniques and apparatus have permitted a significant increase in the utilization of radio communication systems. However, because the transmission channels upon which the modulated signals of the radio communication systems are transmitted are formed of radio frequency channels, and because radio frequency channels are limited in number by the amount of the electromagnetic frequency spectrum allocated for use by various radio communication systems, further increased utilization of various radio communication systems is limited by limitations in the number of the radio frequency channels upon which modulated signals may be transmitted.
Generally, only by increasing the number of frequency channels defined upon a frequency band or by more efficiently utilizing the frequency channels defined upon the frequency band may the communication capacity of a communication system operable upon a particular frequency band be increased.
By converting a communication signal into discrete form prior to transmission thereof, more efficient facilitation of a presently-defined frequency channel is possible as the resultant modulated signal is typically of a smaller modulation spectrum than a corresponding modulated signal comprised of a communication signal that has not been converted into discrete form. And, when the communication signal is converted into discrete form prior to modulation thereof, the resultant, modulated signal may be transmitted in short bursts, and more than one modulated signal may be transmitted sequentially upon a single frequency channel.
As a single frequency channel may be utilized to transmit two or more separate signals during nonoverlapping time periods, a method of signal transmission in which two or more separate signals are transmitted in such manner is oftentimes referred to as a time division method. A communication system incorporating such a time division method of signal transmission includes a Time Division Multiple Access communication system or, more simply, a TDMA communication system.
A TDMA communication system includes a transmitter operative to transmit signals to a receiver in bursts during intermittent time periods. Such signal shall, at times, hereinafter be referred to as a TDMA signal. A TDMA communication system further includes a receiver operative to receive a TDMA signal. The discretely-encoded communication signal transmitted in the intermittent bursts is transmitted during particular time slots defined by the TDMA communication scheme in which the TDMA communication system is operable. The signal transmitted by a transmitter during any particular time slot shall hereinafter be referred to as a sequence.
Other types of communication systems similarly convert a communication signal into discrete form prior to transmission thereof.
Whenever a signal is converted into discrete form and transmitted as sequences of discrete information bits, noise and other interference introduced upon the communication signal during transmission thereof upon the transmission channel results in distortion of the transmitted signal. When a receiver receives the transmitted signal including significant amounts of such distortion, the receiver is unable to decode accurately the received signal in order to recreate the signal actually transmitted by the transmitter. As a result, the audio quality of the received signal, once recreated by the receiver, is degraded.
The receiver oftentimes includes circuitry operative to determine the amount of distortion introduced upon the signal during transmission thereof to the receiver. When such circuitry determines that excessive amounts of distortion are introduced upon portions of the communication signal, the circuitry may be made further operative to cause the receiver not to decode the affected portions of the communication signal. Thereby, degradation of the audio quality of the signal actually recreated by the receiver may be minimized.
Various signalling protocols have been developed which are operative, in conjunction with the circuitry of the receiver, to permit a receiver to determine better whether a signal transmitted thereto includes excessive amounts of noise or other interference introduced thereupon. For instance, a TDMA communication system protocol scheme has been promulgated in Japan in a standard specification for cordless telephone-type apparatus. The protocol scheme includes requirements for formatting sequences of the signal transmitted during the intermittent time periods of a TDMA communication system and also the modulation method in which a modulated signal is formed. Namely, the modulation method is a Differential Quadrature Phase Shift Keying (DQPSK) method.
A portion of the sequence transmitted during the selected time slots includes not only portions of the information signal converted into discrete form, but also predetermined data transmitted for purposes of timing and signal protocol. Included amongst such predetermined data are sequences of selected codes. Corresponding codes are stored in the receiver, and, when the receiver receives a signal transmitted thereto, the receiver compares the received signal with the stored information.
In the above-mentioned Japanese standard specification, such codeword is referred to as a "unique word." When the receiver determines that the unique word received by the receiver differs with the unique word stored by the receiver, the receiver is able to make a determination that excessive amounts of noise or other distortion have been introduced upon a transmitted signal. Use of such a determination alone, however, permits a receiver, in some instances, to determine that a sequence of a received signal does not include excessive amounts of noise or other distortion introduced thereupon when, in fact, excessive amounts of noise or other distortion have been introduced thereupon.
The same above-mentioned, Japanese standard specification further sets forth a second code which forms a portion of the data transmitted by a transmitter with each sequence of information transmitted to the receiver. Such code, referred to as a Cyclic Redundancy Code (CRC), may also be utilized, once received by a receiver, to make a determination as to whether excessive amounts of noise or other distortion have been introduced upon the sequence of the signal during transmission thereof to the receiver. Determination of whether a sequence of a signal transmitted to a receiver includes excessive amounts of noise or other distortion introduced upon the sequence during transmission thereof to the receiver based solely upon analysis of the Cyclic Redundancy Code, however, results in the receiver determining that too many sequences of the transmitted signal include excessive amounts of noise or other distortion. Accordingly, determinations made by the receiver as to whether excessive amounts of noise or other distortion have been introduced upon the sequence of the signal transmitted to the receiver based solely upon analysis of the Cyclic Redundancy Code is also inadequate.
What is needed, therefore, is circuitry for a receiver which permits the receiver to make an accurate determination as to whether or not a portion of a discretely-encoded signal received thereat contains excessive amounts of noise or other distortion introduced thereupon. Such determination would permit degradation of the audio quality of the signal actually recreated by the receiver to be minimized, as the receiver may be caused not to decode portions of the discretely-encoded signal which contain excessive amounts of noise or other distortion.